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Precision

Precision is repeatability. Precision indicates how well a method or instrument gives the same result when a single sample is tested repeatedly. Precision measures the random error of a method, which is the scatter in the data. Precision does not indicate that an instrument is reporting the correct result; which is accuracy.

Two types of precision are measured; within run and between run. Within run precision provides an optimistic estimate of the expected daily performance of a method or instrument since there is minimal opportunity for operating conditions to change during a single analytical run. Within run precision, must be evaluated and accepted before proceeding with more comprehensive studies.

Quality control material is available commercially. At least two, and sometimes three, concentrations of quality control material should be run for each analyte. Concentration of the analyte in the quality control samples should be as close as possible to the upper and lower medical decision points. These decision points could represent the upper and lower reference values or nationally recommended decision points.

Clinical Laboratory Standards Institute recommends running two levels of quality control material three times per run for five different runs, giving 15 replicates of each level. Most in vitro diagnostic companies use this protocol when they install a new instrument in a clinical laboratory.

Some laboratories believe that a good precision study should include 20 to 50 replicates. The larger the number of replicates, the more confident you can be in the precision results. For example, if the true SD of a method is 1.00, a precision estimate based on 20 replicates might range from 0.76 to 1.46. The precision estimate based on 50 replicates is narrower, ranging from 0.84 to 1.24.

Mean, standard deviation (SD), and coefficient of variation (CV) are calculated for each level using a spreadsheet.

  • Mean is the average value, which is calculated by adding the results and dividing by the total number of results.
  • SD is the primary measure of dispersion or variation of the individual results about the mean value. The easiest way to calculate SD is use the statistical tools present in a spreadsheet such as Excel. The greater the imprecision, the larger the standard deviation will be. For many analytes, SD varies with sample concentration. Using glucose as an example, an SD of 10 for a 400 mg/dL sample indicates very good precision, but an SD of 10 for a 40 mg/dL sample represents very poor precision.
  • CV is the SD expressed as a percent of the mean (CV = standard deviation/mean x 100). The higher the standard deviation, the greater the percentage of the mean it becomes and the higher the %CV.
  • The 95% confidence interval for SD is a measure of the precision of the precision estimate. The width of the confidence interval depends on the number of samples analyzed and the intrinsic SD of the method.

If an instrument or method has good precision, 95% of values should fall within 2 standard deviations of the mean. That means that no more than 1 of the 20 results should fall outside of 2 standard deviations.

Calculated SD and CV should be compared to the manufacturer’s published statistics. If the obtained results are higher than the manufacturer’s claim, an investigation must be undertaken before proceeding further with the method evaluation.

Below is an example of within run precision for Level 1 quality control material for sodium. The QC material is repeated 30 times and the following results are obtained: 110, 110, 111, 111, 111, 111, 111, 111, 111, 111, 111, 111, 111, 112, 112, 112, 112, 112, 112, 112, 112, 112, 112, 112,112,112, 113, 113, 113, and 113.

The sum of the thirty results is 3346. The mean is calculated by dividing 3346 by 30, which gives a result of 111.6. SD is 0.8 and CV is 0.72%. The 95% confidence interval is the mean +/-2SD or 110.0 – 113.2.

Between run precision is a better indicator of a method’s overall precision than within run precision because it measures the amount of random error inherent in the method from day to day. Between run precision is affected by many variables such as changes in operators, reagents and ambient operating conditions.

Concentration of the analyte in quality control samples should be as close as possible to the upper and lower medical decision points (usually the reference limits). Between run precision should be evaluated over at least 20 days using at least 2 reagent lots. The mean, standard deviation, and coefficient of variation are calculated for each level.

The standard deviation obtained during day-to-day replication studies is expected to be greater than the standard deviation of within run studies. The maximum allowable between run standard deviation is a matter of judgment. Generally, it should be less than total allowable error (see Appendix B).

Acceptable CVs need to be defined for each analyte based on medical significance. Generally, precision should be equal to or less than one half of the within subject biological variation. Desirable precision levels for some common chemistry analytes are summarized in the following table.

Analyte CV % Analyte CV%
Chloride 0.6 Cholesterol 3.0
Osmolality 0.7 Free T4 3.8
Calcium 1.0 Phosphate 4.3
Albumin 1.6 AST 6.0
Magnesium 1.8 TSH 9.7
Creatinine 2.2 Triglycerides 10.5
Potassium 2.4 ALT 12.2

Precision levels vary depending on the analyte and the method. Generally, electrolytes and creatinine have very low CV% indicating very good precision. Enzymes and immunoassays typically have higher CV%. One other rule of thumb is that a method’s CV or SD should be

Precision indicates how closely an instrument reproduces the same result when tested repeatedly. Precision is important for serial monitoring of lab tests.

South Dakota Presidential Election Results

Updated Nov. 28, 2020, 4:42 AM ET

South Dakota Presidential Election Results

Donald J. Trump wins South Dakota.

Race called by The Associated Press.

Nearly all of the estimated vote total has been reported.

Results by county

Ziebach and Lyman Counties have shifted left, toward Biden, compared with 2016.

Note: Absentee vote data may not be available in some places.

Tracking the vote count

See how the reported vote share changed over time.

Absentee votes by candidate

Some states and counties will report candidate vote totals for mail-in ballots, but some places may not report comprehensive vote type data.

Candidate Absentee/early votes Votes Pct.
Trump
Biden
Jorgensen
Total reported

0% of counties (0 of 66) have reported absentee votes. Data for absentee votes may not be available in some places.

Latest updates

Michael D. Shear, in Washington Nov. 23, 2020

President Trump authorized his government to begin the transition to President-elect Joseph R. Biden Jr.’s administration. Read more ›

Read our analysis of the vote

Latest updates

Michael D. Shear, in Washington Nov. 23, 2020

President Trump authorized his government to begin the transition to President-elect Joseph R. Biden Jr.’s administration. Read more ›

Kathleen Gray, in Bloomfield Hills, Mich. Nov. 23, 2020

Michigan’s top elections board voted to certify the election results, a blow to President Trump, who had been trying to subvert Joe Biden’s win there. Read more ›

Richard Fausset, in Atlanta Nov. 20, 2020

Georgia’s secretary of state has certified President-elect Joseph R. Biden Jr.’s victory in the state, dealing a blow to President Trump’s bid to overturn the election. Read more ›

Richard Fausset, in Atlanta Nov. 19, 2020

President-elect Joseph R. Biden Jr.’s victory in Georgia was reaffirmed after the state finished its recount of nearly five million ballots with few meaningful vote changes. Read more ›

Stephanie Saul, in New York Nov. 13, 2020

Biden is the first Democratic presidential candidate to carry Georgia since Bill Clinton in 1992. Even as a recount begins in the state, Biden leads by more than 14,000 votes. See Georgia results ›

Matt Stevens, in New York Nov. 13, 2020

Trump’s narrow victory in North Carolina does not affect the overall outcome of the race, which Biden won Saturday after crossing the threshold of 270 Electoral College votes. See North Carolina results ›

Nate Cohn, in New York Nov. 13, 2020

It’s final: Joe Biden wins 306 electoral votes, Donald Trump wins 232. Biden wins Georgia. Trump wins North Carolina. Read more ›

Jennifer Medina Nov. 12, 2020

For the first time in decades, Arizona has voted for a Democrat for president. Bill Clinton won the state in 1996, and he had been the only Democrat to do so since Truman. Read more ›

Nate Cohn, in New York Nov. 12, 2020

The races in Arizona and Georgia have not been called by several TV networks, but they’re essentially over. Here’s why ›

Nicholas Fandos Nov. 11, 2020

Senator Thom Tillis of North Carolina capitalized on unexpected Republican strength in a crucial swing state to defeat a Democrat damaged by revelations of an extramarital affair. Read more ›

Maggie Astor, in New York Nov. 11, 2020

Biden’s popular vote lead has surpassed five million, putting him 3.4 percentage points ahead of Trump. This is significantly larger than Hillary Clinton’s 2.9 million, 2.1-point margin in 2016.

Glenn Thrush, in Washington Nov. 11, 2020

Georgia will conduct a hand recount, a move requested by the Trump campaign. State officials have said it is unlikely to erase Biden’s narrow but significant lead there. Read more ›

Carl Hulse, in Washington Nov. 11, 2020

Senator Dan Sullivan’s victory in Alaska moves Republicans closer to holding the Senate. Two Georgia runoffs will decide Senate control. Read more ›

Source: Election results from National Election Pool/Edison Research

By Michael Andre, Aliza Aufrichtig, Gray Beltran, Matthew Bloch, Larry Buchanan, Andrew Chavez, Nate Cohn, Matthew Conlen, Annie Daniel, Asmaa Elkeurti, Andrew Fischer, Josh Holder, Will Houp, Jonathan Huang, Josh Katz, Aaron Krolik, Jasmine C. Lee, Rebecca Lieberman, Ilana Marcus, Jaymin Patel, Charlie Smart, Ben Smithgall, Umi Syam, Rumsey Taylor, Miles Watkins and Isaac White

Additional data collection by Alice Park, Rachel Shorey, Thu Trinh and Quoctrung Bui

Candidate photo research and production by Earl Wilson, Alana Celii, Lalena Fisher, Yuriria Avila, Amanda Cordero, Laura Kaltman, Andrew Rodriguez, Alex Garces, Chris Kahley, Andy Chen, Chris O’Brien, Jim DeMaria, Dave Braun and Jessica White

See full results and maps for the 2020 presidential election in South Dakota. ]]>